The present invention relates generally to neurological magnetic resonance (MR) imaging systems, and more particularly, to an apparatus and system for MR scanning and imaging of a cerebrum.
In neurological magnetic resonance imaging, a patient is typically positioned within a strong, temporally constant magnetic field. A time series of magnetic field gradient pulses, for encoding spatial location, are applied across a region of interest within the magnetic field. Concurrently, radio frequency pulses are applied for inducing and manipulating magnetic resonance of dipoles in the region of interest. A system of radio frequency transmitting and receiving coils is positioned over the head and neck portions of a patient to excite and receive radio frequency magnetic resonance signals within the region of interest.
Several types of radio frequency coils are utilized in the art, including coils of a “Birdcage Resonator” type, of a Transverse Electromagnetic (TEM) type, and of a “Dome Resonator” type. Each of these resonators, when suitably configured, can act as a transmitter and/or receiver for exciting and/or receiving magnetic resonance within a selected volume of interest, as is known in the art.
Coils of the birdcage resonator type are typically in the form of a ladder circuit, which may be of high pass or low pass type. The ladder circuit is typically of cylindrical form and has axially-directed runners extending between a pair of coaxial conductive rings, sometimes referred to as “end rings”, located at the peripheries of the cylinder, and extending azimuthally thereabout. In the low pass birdcage type, each runner has a given number of capacitors; in the high pass birdcage type, each ring segment joins two runners and has a given number of capacitors.
A birdcage coil may or may not be fitted with a cylindrical conductive shield, surrounding, but typically not touching, the resonant structure; the stand off distance between shield and resonator is a design variable. The function of the shield is twofold: i) to reduce electromagnetic couplings to surrounding conductive objects, which are harmful since they can detune the resonator or constitute an extraneous source of energy losses, and ii) to cancel or reduce radiative losses from the resonator at sufficiently high frequencies. The benefits of shielding are typically realized in resonators operating at rather high radio frequencies that are often above 100 MHz.
TEM coils are similar to coils of the shielded birdcage type, in that they are cylindrical, and have axially directed longitudinal runners; but these runners are not conductively connected to end rings. Instead, the runners are connected to the outer cylindrical shield via distributed or discrete capacitance. Inasmuch as the runners themselves constitute typically an inductive impedance, the circuit of each runner is connected capacitively to the shield and may be viewed as a low pass pi circuit over a ground plane.
Standard geometry of a coil of the shielded birdcage resonator type and of the TEM type provide a uniform RF B field, but can be claustrophobic in nature and are inefficient for imaging of just a cerebrum, since they bound a relatively large volume as compared to coils of the dome resonator type, which contain comparatively less tissue. That is, since the principle loss mechanism in imaging resonators is from radiofrequency eddy currents, due to the conductivity of the specimen being imaged, lowering the amount of tissue within the resonator also lowers the losses and leads to a more efficient resonator.
The birdcage and the TEM coils are inefficient due to their large size, which can cover a larger portion of a patient than is usually of interest. Also, the birdcage and the TEM coils experience greater than necessary power dissipation and usually acquire data from regions of a body that are not necessarily of interest.
The dome resonator coil has not achieved wide usage and is defined as a radio frequency (RF) resonator that includes conductors and capacitors arrayed upon an outer surface of a hollow dome-shaped lamina, such as commonly referred to as a surface of revolution, for example a paraboloid. The dome resonator is usually employed for its high sensitivity, despite its poor RF homogeneity, which has tended to restrict its use to pure reception as opposed to being used for transmission. Similar to the birdcage coil design, the dome resonator coil includes longitudinal runners that extend between an aperture ring and an apex. Each runner, such as in a low-pass dome resonator type, has a given number of capacitors.
The dome resonator coil is intended typically, though not exclusively, to serve as an antenna for MR imaging and spectroscopy of a human cerebrum or brain. The hollow dome shape of the dome resonator coil is designed so that its interior bounds a volume approximating the shape of a human head, which is intended to fit therein.
Coils of the dome resonator type are streamlined, patient friendly, and efficient, but produce a non-uniform RF B field. The non-uniform B field increases in strength closer to the apex, due to bunching or close proximity of the conductive runners and concomitant bunching of magnetic flux lines. The increase in field strength near the apex causes the field to be non-uniform and thus degrades image quality.
In addition to the above stated and associated disadvantages of the existing RF coil devices, the RF resonators are, as noted above, more and more likely to be subject to parasitic electromagnetic couplings to surrounding metallic objects as well as to radiation losses. This is especially true when MR imaging and spectroscopy systems operate at increased static field strengths and use higher frequencies.
It is desirable to operate within increased static field strengths, such as 3.0 T and 4.0 T, instead of the conventional 1.5 T. Unfortunately, due to electrical properties of a patient, artifacts are increased at the higher static field strengths. However, rapid pulsing and non-zero relaxation rates tend to mitigate these artifacts and produce a leveled image, provided that the distortion is not originally overly severe. The artifacts derive from the fact that a human head has electrical properties such that it is a lossy ellipsoid of high dielectric constant, which produces resonant effects at radio frequencies required for performance of MR imaging and spectroscopy at static fields above 3.0 tesla. The resonant effects distort uniformity of the RF excitation. The result is the production of artifacts in the form of distorted image intensities, which are typically brightened at the centers of the images.
It is therefore desirable to provide an RF receiving coil device that has the advantages of both the birdcage/TEM type coils and the dome resonator type coil without the above-stated disadvantages and at the same time is less susceptible to higher frequencies and static field strengths.